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3.1 | |
EIGRP Concepts | |
| 3.1.1 | |
Comparing EIGRP with IGRP |
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Cisco released EIGRP in 1994 as a
scalable, improved version of its proprietary distance vector routing
protocol, IGRP. The same distance vector technology found in IGRP is
used in EIGRP, and the underlying distance information remains the same.
EIGRP improves the convergence properties and the operating efficiency significantly over IGRP. This allows for an improved architecture while retaining the existing investment in IGRP. Comparisons between EIGRP and IGRP fall into the following major categories:
IGRP and EIGRP are compatible with each other. This compatibility provides seamless interoperability with IGRP routers. This is important so users can take advantage of the benefits of both protocols. EIGRP offers multiprotocol support, but IGRP does not. EIGRP and IGRP use different metric
calculations. EIGRP scales the metric of IGRP by a factor of 256. That
is because EIGRP uses a metric that is 32 bits long, and IGRP uses a
24-bit metric. By multiplying or dividing by 256, EIGRP can easily
exchange information with IGRP.
IGRP has a maximum hop count of 255. EIGRP has a maximum hop count limit of 224. This is more than adequate to support the largest, properly designed internetworks. Enabling dissimilar routing protocols
such as OSPF and RIP to share information requires advanced
configuration. Redistribution, the sharing of routes, is automatic
between IGRP and EIGRP as long as both processes use the same autonomous
system (AS) number. In Figure
EIGRP will tag routes learned from IGRP or any outside source as external because they did not originate from EIGRP routers. IGRP cannot differentiate between internal and external routes. Notice that in the
show ip route
command output for the routers in Figure
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, RTB
automatically redistributes EIGRP-learned routes to the IGRP AS, and
vice versa.
, EIGRP
routes are flagged with D, and external routes are denoted by EX. RTA
identifies the difference between the network learned via EIGRP
(172.16.0.0) and the network that was redistributed from IGRP
(192.168.1.0). In the RTC table, the IGRP protocol makes no such
distinction. RTC, which is running IGRP only, just sees IGRP routes,
despite the fact that both 10.1.1.0 and 172.16.0.0 were redistributed
from EIGRP.
